The present invention relates to a method of transmitting data signals in a communication system with access to the transmission medium organized on a central or distributed basis using a number of transmission modes. In particular the present invention relates to a signaling scheme for adaptive modulation in a CSMA/CA (Carrier Sense Multiple Access Collision Avoidance) IP-based access system.
A serious problem in mobile radio transmission is the frequency selectivity of the mobile radio channels. The frequency selectivity, generated by multipath propagation with large delay time differences, causes heavily linear distortions of the receive signal, which makes it necessary to use expensive equalizers or to use Viterbi detection. A suitable way of combating the disadvantages of frequency selective channels is what is known as Adaptive Modulation (AM), which is described in greater detail below.
Adaptive Modulation is used in OFDM (Orthogonal Frequency Division Multiplexing) systems to reduce the disadvantages of frequency selective fading channels. In such cases, the data is transmitted over individual subcarriers.
A brief description of the principle of adaptive modulation will be given below. A transmitter transmits data to a receiver over the radio channel. In the transmitter, the data to be transmitted is initially coded and interleaved by a coder and an interleaver. Subsequently, the data is modulated with a different modulation loading, depending on the channel attribute. Suitable modulation alphabets/methods for this are, for example, the known amplitude/phase shift keying methods BPSK, QPSK, 16 QAM, 64 QAM, etc. with the relevant modulation loadings 1, 2, 4 and 6. With a high signal-to-noise ratio the relevant subcarrier is to be modulated with a high number of bits whereas with a low signal-to-noise ratio a low number of bits is sufficient. The signal-to-noise ratio normally will be estimated in the receiver and converted for the individual subcarriers in a bit loading table. A bit loading table of this type can, for example, contain information about the signal-to-noise ratio or alternatively the required modulation loading for each individual subcarrier. This bit loading table is transferred to the transmitter so that the latter can activate a demultiplexer DEMUX and a multiplexer MUX, accordingly, for adaptive modulation.
The demultiplexer DEMUX directs the bit stream received by the interleaver to the modulator MOD1, . . . MODn-1, MODn, assigned a specific modulation loading. In this case, modulator MOD1 can, for example be a BPSK modulator and modulator MODn a 64 QAM modulator. Depending on the pointers received by the relevant modulation, the multiplexer MUX which is also controlled via the bit loading table is subjected to an inverse Fast-Fourier Transformation IFFT. The pointers to the relevant subcarriers are transformed there for transmission and subsequently modulated up to the carrier frequency.
In the receiver this process is essentially reversed. Initially, the data is obtained as pointers from the individual subcarriers using a Fast-Fourier transformation. A subsequent demultiplexer DEMUX allocates the data in accordance with the bit loading table to the suitable demodulator. The bit stream obtained from the demodulators DEMOD1 . . . , DEMODn-1, DEMODn is fed via a multiplexer MUX to a deinterleaver and channel coder.
As already mentioned, the desired bit loading table for adaptive modulation is to be transmitted from the transmitter to the receiver. An important point here is that the bit loading tables are typically calculated in the receiver on the basis of RSSI (Radio Signal Strength Indication) and SNR (Signal to Noise/Interference Ratio) and must be transferred to the transmitter. For a TDD (Time Division Duplex) scheme a WSS (White Sense Stationary) channel is usually assumed for a period in which the bit loading table is valid.
Standard IEEE 802.11a specifies the Medium Access Control (MAC) and physical characteristics for radio LAN systems. A Medium Access Control unit in accordance with this Standard is designed to support the components of a physical layer depending on the availability of the spectrum as regards their permission to access the transmission medium.
Basically, there are two coordination options for access available: the central and the local access function. With the central access function (Point Coordination Function, PCF) the coordination function logic is only active in a station or in a terminal of a group of terminals (Basic Service Set, BSS), respectively, for as long as the network is in operation. By contrast, with the Distributed Coordination Function (DCF) the same coordination function logic is active in each station or each terminal of the terminal group, respectively, for as long as the network is in operation.
FIG. 1 shows the data frame structure for this exchange of data in a Distributed Coordination Function (DCF) according to Standard IEEE 802.11. The reader is referred to this Standard as regards the abbreviations and terms used in this document. According to FIG. 1, the following units participate in communication: a transmitter, a receiver and other units. After a wait time, known as the DCF Interframe Space (DIFS), the transmitter transmits an RTS (Ready to Send) signal to the network.
After a short wait time (Short Interframe Space, SIFS), the receiver sends the CTS(Clear to Send) signal, indicating that it is ready to receive. After another short wait time SIFS the transmitter sends to the network the data to be transmitted. After the transmission and a wait time SIFS, the receiver confirms the receipt of the data with the acknowledge message ACK. The wait times SIFS and DIFS in this case are 16 μs and 34 μs, respectively.
For other communication users the NAV (Network Allocation Vector) is set at the initiative of the RTS or CTS signal, specifying for how long a transmission cannot be executed on the wireless medium by the relevant station.
Access to the radio system is only possible again once the wait time DIFS has elapsed after receipt of the acknowledgement ACK of the receiver. In the subsequent window, known as the contention window, to avoid collisions, there is a delay by a random backoff time.
FIG. 2 shows the frame or data packet formats of the frames shown in FIG. 1. Of particular importance in this context is the interplay between transmitter and receiver and, thereby, the relevant addressing. Thus, the RTS frame accommodates the Transmitter Address (TA) encoded with six octets. The Receiver Address is also coded with six octets in the CTS frame. The data frame which is sent by the transmitter contains the destination address in address block “Address 2.” The ACK frame returned by the receiver for acknowledgement again contains the RA (Receiver Address) so that the transmitter can uniquely assign the acknowledgement.
An object of the present invention is to increase the channel capacity in a communication system.